O-rings are often used for sealing an annular gap between two close-fitting members. Typically the O-ring is seated in an annular groove formed in one of the two close-fitting members. The close-fitting members are often nested tubular members carrying fluid or air under pressure, and in this case the annular groove is most often formed in an inner one of the nested tubular members.
It is often desirable for the O-ring to be made of an elastomeric material in order to seal an annular gap. However, elastomeric materials that are sufficiently elastic and compliant to accommodate large variations in gap width are typically subject to extrusion under high pressure fluid or air. Therefore this extrusion will limit the fluid pressure that can be contained by the O-ring. When it is desired for an O-ring to contain a high pressure fluid and accommodate a significant gap, a backup ring is often used in conjunction with the O-ring to reduce extrusion of the O-ring into the gap and therefore increase the maximum fluid pressure that can be contained by the O-ring.
In use, the backup ring is placed on the low-pressure side of the O-ring, in the same annular groove holding the O-ring. The backup ring is made of a material that is resilient and less compliant but more durable than the O-ring. For example, the O-ring is made of soft rubber, and the backup ring is made of resilient metal. Thus, the backup ring can reduce or close the gap without necessarily sealing the gap, in order to limit extrusion of the O-ring into the gap, so that the O-ring will seal the gap and contain a higher fluid pressure than would be possible without the backup ring.